1. Field of the Invention
The present invention relates to QRS wave filters for use in a heart monitoring ECG system, and particularly, for use in an NMR imaging system.
2. Description of the Prior Art
QRS wave filters are known. A QRS wave is an important component of the electrical activity of the heart and corresponds to end diastole. For example, an article by Thakor et al. "Estimation of QRS Complex Power Spectra for Design of a QRS Filter" IEEE Transactions on Biomedical Engineering, Vol. BME-31, No. 11, November 1984, pages 702-706, discusses filtering ECG waveforms presenting spectral analysis of ECG waveforms. They indicate that the QRS complex can be separated from other interfering signals. They disclose that a bandpass filter would be of use in many ECG monitoring instruments. A particular filter is disclosed. Disclosed as interference are noise signals due to motion artifacts, muscles and 60 Hz powerline noise. However, this filter is not particularly useful in a Nuclear Magnetic Resonance imaging (NMR) system.
In an NMR system the ECG monitoring probes and wires need be placed on the chest of a patient. Such wires move about due to the breathing of the patient. The present inventor recognizes a need for an ECG filter which will filter breathing induced noise on the wires in the presence of a static magnetic (Bo) field. There is no recognition in the Thakor et al. article of the nature of such noise in which the motion of the wires in the Bo field cause currents, i.e., noise, to be induced in the ECG wires. Further, another problem recognized by the present inventor is that it would be desirable to provide an ECG signal in real time with an image of the heart. In this way a pathological malfunction can be visually observed on a monitor simultaneously with the QRS Wave. Timing is extremely important for the QRS wave and image to enhance correct diagnosis of the pathology. The Thakor article bandpass filter also does not allow for such timing and does not address the problem of such a delay in the filtered signal which may cause the visual occurrence of a pathological event to be out of phase with a pathological ECG event. Correlation of these out of phase events becomes difficult because different pathologies of the heart cause different pathologies in the QRS wave, which due to an out of phase condition, may cause misdiagnosis of the condition.
Another article on ECG QRS wave filtering is "R-Wave Detection in the Presence of Muscle Artifacts" by De Vel, IEEE Transactions on Biomedical Engineering, Vol. BME-31, No. 11, November 1984, pages 715-717. This discloses a filter and filtering for noises similar to those discussed in the Thakor article. In particular, the article deals with motion and muscle caused artifacts as well as other electrical perturbations. The disclosed filter, however, exhibits a 300 ms delay as shown in FIG. 3a thereof. This delay is not acceptable in an NMR imaging system for the reasons given above. Neither article recognizes the need for a filter in an NMR imaging system nor the nature of the noises created by such a system on the QRS signal. For example, the bandwidth of the Bo field induced noises relative to the QRS signal is not disclosed, nor more importantly, the need for time correlation of the NMR produced image with the ECG produced QRS wave.